1. Field of the Invention
This invention relates to the field of thermal insulation coverings. More particularly, this invention relates to encasing materials for wrapping about an insulating material.
2. Description of the Prior Art
It is known that pipes, equipment, and other objects can be thermally insulated, usually with mineral fibers, and that this insulating material can be protected against external influences, for example, weather, by means of a sheet-metal covering.
For this purpose, galvanized sheet iron as well as aluminum sheet has been used. The aluminum sheet is somewhat more desirable than the galvanized sheet metal since aluminum possesses both endurable weathering resistance as well as a greater ability to reflect heat radiation. However, the sensitivity of aluminum toward alkali is a distinct disadvantage of this material.
Consequently, corrosion damage can occur due to the action of moisture when an alkaline reacting insulating material, e.g., calcium silicate, is used. Corrosion damage can also be observed when the aluminum comes into contact with other metals, for example, the baling wire on the insulation, which results in the formation of local electrolytic cells.
The damaging effect of the alkali, as well as the formation of the local electrolytic cells, can be avoided by enveloping or wrapping the insulation with a plastic film. However, this incurs additional costs.
A significant improvement in such encasing material was achieved by furnishing the reverse side of the aluminum sheet with a barrier for protection against corrosion by laminating a polyethylene coated kraft paper web, i.e., the side of a kraft paper web which was coated with polyethylene film on one side. However, such a laminate has the disadvantage that the high heat-radiation reflecting ability of the aluminum on the reverse side is lost due to the insulating system. Also, the additional cost of such an insulating sheeting due to the lamination on the reverse side has limited its application to calcium chloride insulation material. This type of material is conventionally used in the United States due to the natural occurrence of calcium silicate. Thus, the insulating material obtained from calcium silicate is so mechanically stable that the thickness of the aluminum sheet can be substantially reduced so that the aluminum sheeting which is furnished on the reverse side of the above-described lamination is sufficiently thin as to be economically feasible.
However, in Europe and particularly, in Germany, the most readily available insulating material is mineral fibers, such as, for example, glass wool or basalt stone wool. This material possesses a much looser construction than the calcium chloride insulating material. Consequently, the insulating sheeting which is used for this type of material must possess substantially greater mechanical stability and, must be correspondingly thicker.
For economic reasons, an aluminum sheet having a thickness required by the mechanical stability considerations cannot also be furnished on its reverse side with the above-described corrosion barrier although this would be advantageous in view of the danger of pitting corrosion.